Re: smr inp breaks some jail use cases and panics with i915kms don't switch to the console anymore

  Hi John,

On Mon, Dec 13, 2021 at 07:45:07AM -0800, John Baldwin wrote:
J> So there are two things here.  The root issue is that the devel/apr1 port
J> runs a configure test for TCP_NDELAY being inherited by accepted sockets.
J> This test panics because prison_check_ip4() tries to lock a prison mutex
J> to walk the IPs assigned to a jail, but the caller (in_pcblookup_hash()) has
J> done an smr_enter() which is a critical_enter():

The first one is known, and I got a patch to fix it:

https://reviews.freebsd.org/D33340

However, a pre-requisite to this simple patch is more complex:

https://reviews.freebsd.org/D33339

There is some discussion on how to improve that, and I decided to do that
rather than stick to original version. So I takes a few extra days.

We could push D33340 into main, if the negative effects (raciness of
the prison check) is considered lesser evil then potentially contested
mtx_lock in smr section.

J> However, it was a bit harder to see this originally as the 915kms driver
J> tries to do a malloc(M_WAITOK) from cn_grab() when entering DDB which
J> recursively panics (even a malloc(M_NOWAIT) from cn_grab() is probably a
J> bad idea).  When it panicked in X the result was that the screen just froze
J> on whatever it had most recently drawn and the machine looked hung.  (The
J> fact that that sysbeep is off so I couldn't tell if typing in commands was
J> doing anything vs emitting errors probably didn't improve trying to diagnose
J> the hang as "sitting in ddb" initially, though I don't know if DDB itself
J> emits a beep for invalid commands, etc.)

Didn't know about this one. Is this isolated to actually entering DDB or
there is some path that in a normal inpcb lookup we would M_WAITOK?

-- 
Gleb Smirnoff